Each year fires destroy millions of acres of woodland, lives, and property, and significantly contribute to air pollution. Increased knowledge of the physics and properties of the flame propagation is necessary to broaden the fundamental understanding and modeling capabilities of fires. Modeling flame propagation in fires is challenging because of the various modes of heat transfer with diverse fuels, multi-scale turbulence, and complex chemical kinetics. Standard physical models of turbulence like RANS and LES have been used to understand the flame behavior, but these models are limited by computational cost and their inability to resolve sub-grid scales. Application of several other models and empirical studies in fire modeling are usually limited to fire spread rate only. In some fires, flame propagation often occurs through convective heating by direct flame contact as opposed to radiative preheating alone. Under these conditions, resolution of the flame front can provide the detailed physics and insights into the flame propagation. The One Dimensional Turbulence (ODT) model is extended to turbulent flame propagation in biomass fuel beds representative of those in wild land fires. ODT is a stochastic model that is computationally affordable and can resolve both large and fine scales. ODT has been widely applied to many reacting and non-reacting flows like jet flames and pool fires. A detailed particle combustion model has been developed and implemented in the ODT model to investigate the fluctuating flame-fuel interface and to study flame propagation properties. The particle reaction is modeled as a single global decomposition reaction model. Radiative, convective, and internal particle conductive heat transfer are included. Gaseous combustion is modeled with a lookup table parameterized by mixture fraction and fractional heat loss using steady laminar flame let solutions. Results are presented from simulations of flame propagation in buoyantly driven flows. Particle size and loading are varied to study their effects in flame spread. A timescale analysis is performed to compare radiative, convective, conductive, and reactive particle time scales to the turbulent fluctuations. The flame propagation in homogeneous turbulence is also studied which better represents the wildland fire. The time scales involved in the wildland fire are overlapped using LEM model to study their effects on the flame properties and flame spread.

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In gas stirred steelmaking ladles, entrainment of slag into metal and vice versa takes place. The slag entrainment has been shown to abruptly increase the mass transfer rates of refining reactions through high temperature and water modeling studies of the past. However such an effect has not been correlated with the degree of entrainment, since the latter has not been quantified in terms of operating parameters like gas injection rate and fluid properties. Much of the past works are limited to finding the critical conditions for onset of entrainment. The difficulty lies in measuring the degree of entrainment in industrial ladles or even in a water model. Mathematical modeling is also challenging due to the complexity of the multiphase phenomena. So in this thesis, a modular mathematical modeling approach is presented wherein the phenomena of slag entrainment into metal is resolved into four aspects, models developed for each and finally integrated to study its role. The individual models are (1) multiphase large eddy simulations to simulate slag entrainment in a narrow domain that receives its boundary conditions from (2) single phase RANS simulation of a full ladle, (3) a Lagrangian particle tracking method to compute the residence times of slag droplets in metal phase and (4) a kinetic model that integrates the above three models to compute mass transfer rate as a function of degree of entrainment. Mass transfer rate predictions comparable to a literature correlation were obtained. This supports the modeling approach and also the assessment of role of various system parameters on entrainment characteristics. In essence, the present work shows a systematic approach to model and study the complex multiphase phenomena. / Thesis / Doctor of Philosophy (PhD) / The entrainment of liquid slag into liquid steel in gas stirred-steelmaking ladles is known to increase the rate of refining drastically. However, there is lack of correlation between degree of entrainment and ladle operating conditions, which this thesis addresses through mathematical modeling.

[ES] El objetivo de esta tesis es la investigación y análisis de la estructura interna de los chorros diésel reactivos y el efecto de las condiciones de contorno en los parámetros asociados a la combustión. Este objetivo se consigue por medio de la simulación numérica del chorro con modelos de turbulencia RANS y LES usando un modelo de combustión avanzado basado en el concepto flamelet. Para este estudio, se aplica una aproximación simplificada de las flamelets de difusión, conocidas en la literatura como Flamelets de Difusión Aproximadas (ADF en inglés), como fundamento del modelo de combustión. En una primera etapa, el modelo se valida con combustibles de diferente complejidad química en regímenes estacionarios y transitorios para el conjunto de posibles velocidades de deformación. Una vez se confirma su idoneidad para condiciones encontradas en chorros diésel, se aplica a la simulación del chorro A del Engine Combustion Network (ECN), representativo de chorros diésel. Para proporcionar un cuadro completo de los fenómenos subyacentes, la combustión se analiza inicialmente para condiciones homogéneas y llamas laminares para las distintas condiciones de contorno de este experimento. Después este análisis se complementa con la simulación de diferentes mecanismos químicos para determinar cómo las características del encendido predichas por el esquema de oxidación afectan a la propagación de llama. Los resultados obtenidos en esta etapa se enlazan con el análisis del chorro turbulento en el contexto de simulaciones RANS y LES para describir cómo el fenómeno de la combustión se modifica con los diferentes niveles de complejidad física. La estructura del chorro turbulento se describe profundamente para las distintas condiciones de contorno y mecanismos químicos en términos de mezcla y escalares reactivos para las fases temporales y las regiones espaciales de la llama. La satisfactoria concordancia con los resultados experimentales muestran que el concepto flamelet, y más particularmente el modelo ADF, es adecuado para las simulaciones de chorros diésel. / [CA] L'objectiu d'esta tesi és la investigació i anàlisi de l'estructura interna dels dolls dièsel reactius i l'efecte de les condicions de contorn en els paràmetres associats a la combustió. Este objectiu s'aconsegueix per mitjà de la simulació numèrica del doll amb models de turbulència RANS i LES usant un model de combustió avançat basat en el concepte flamelet. Per a este estudi, s'aplica una aproximació simplificada de les flamelets de difusió, conegudes a la literatura com Flamelets de Difusió Aproximades (ADF en anglés), com a fonament del model de combustió. En una primera etapa, el model es valida amb combustibles de diferent complexitat química en règims estacionaris i transitoris per al conjunt de possibles velocitats de deformació. Una vegada es confirma la seua idoneïtat per a condicions trobades en dolls dièsel, s'aplica a la simulació del doll A del Engine Combustion Network (ECN), representatiu de dolls dièsel. Per a proporcionar un cuadre complet dels fenòmens subjacents, la combustió s'analitza inicialment per a condicions homogènies i flames laminars per a les distintes condicions de contorn d'aquest experiment. Després esta anàlisi es complementa amb la simulació de diferents mecanismes químics per a determinar com les característiques de l'encesa predites per l'esquema d'oxidació afecten la propagació de flama. Els resultats obtinguts en esta etapa s'enllacen amb l'anàlisi del doll turbulent en el context de simulacions RANS i LES per a descriure com el fenomen de la combustió es modifica amb els diferents nivells de complexitat física. L'estructura del doll turbulent es descriu profundament per a les distintes condicions de contorn i mecanismes químics en termes de mescla i escalars reactius per a les fases temporals i les regions espacials de la flama. La satisfactòria concordança amb els resultats experimentals mostren que el concepte flamelet, i més particularment el model ADF, és adequat per a les simulacions de dolls dièsel. / [EN] The objective of this thesis is the investigation and analysis of the internal structure of diesel-like reacting sprays and the effect of boundary conditions on combustion related parameters. This objective is achieved by means of the numerical simulation of the spray with RANS and LES turbulence models using an advanced combustion model based on the flamelet concept. For this study, a simplified approach for diffusion flamelets, known in the literature as Approximated Diffusion Flamelet (ADF), is applied as the basis of the combustion model. In a first step, this model is validated for fuels with different chemical complexity in steady and transient regimes for the whole set of possible strain rates. Once its suitability is confirmed for conditions found in diesel sprays, it is applied to the simulation of spray A from the Engine Combustion Network (ECN), representative of diesel-like sprays. In order to provide a complete picture of the underlying phenomena, combustion is initially analysed in homogeneous conditions and laminar flames for the different boundary conditions of this experiment. Later, this analysis is complemented with the simulation of different chemical mechanisms in order to determine how the ignition characteristics predicted by the oxidation scheme affect to the flame propagation. The results obtained at this stage are connected with the analysis of the turbulent spray in the context of RANS and LES simulations as a way to track how combustion phenomenon is modified at the different levels of physical complexity. The turbulent spray structure is thoroughly described for the different boundary conditions and chemical schemes in terms of mixing and reactive variables for both temporal phases and spatial flame regions. The satisfactory agreement with experimental results shows that the flamelet concept, and more particularly the ADF model, is suitable for diesel-like sprays simulations. / Pérez Sánchez, EJ. (2019). Application of a flamelet-based combustion model to diesel-like reacting sprays [Tesis doctoral]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/117316

L'objectif de la présente étude est d'analyser la modélisation de la turbulence de simulations en moyenne de Reynolds (RANS) dans le cadre d'écoulements de type turbomachines, en utilisant des simulations aux grandes échelles (SGE) comme référence. L'étude porte sur deux cas test: un décollement de coin dans une grille d'aubes rectiligne, et un écoulement de jeu pour un aubage isolé dans un jet. Deux lois de comportement, la loi de comportement de Boussinesq et la loi de comportement quadratique (quadratic constitutive relation ou QCR), sont analysées, avec deux versions du modèle de turbulence k-omega de Wilcox. Les lois de comportement étudiées reposent sur deux hypothèses: une hypothèse d'alignement entre le tenseur de Reynolds et un tenseur construit à partir de l'écoulement moyen, et une hypothèse sur la viscosité turbulente. L'hypothèse d'alignement est étudiée à partir de la SGE, pour laquelle les deux tenseurs sont indépendamment connus, en utilisant un indicateur construit sur le produit scalaire des tenseurs. Les résultats sont présentés sous forme d'une fonction de répartition de la valeur de l'indicateur pour le domaine complet, puis pour trois sous-domaines d'intérêt: l'entrée, une région où l'écoulement interagit fortement avec les parois, et une région où l'écoulement est fortement tourbillonnaire. L'hypothèse d'alignement n'est que rarement valide pour la loi de comportement de Boussinesq. Pour la QCR, les résultats sont meilleurs en entrée, comparé à la loi de Boussinesq. Il ne sont cependant pas meilleurs pour les régions où l'écoulement est plus tourbillonnaire. Une amélioration de la loi de comportement est nécessaire pour pouvoir faire progresser la modélisation turbulente en RANS. En revanche, l'utilisation de l'énergie cinétique turbulente et du taux de dissipation spécifique semble correcte pour estimer la valeur de la viscosité turbulente. L'analyse de la modélisation de l'équation d'énergie cinétique turbulente (ECT) est réalisée au travers d'une comparaison terme à terme avec l'équation d'ECT résolue par la SGE. Les résultats SGE présentent une turbulence qui n'est pas à l'équilibre : la production et la dissipation ne sont pas superposées, et le terme de transport est important. Pour le RANS, la turbulence est à l'équilibre : la production et la dissipation sont superposées, et le terme de transport est de faible intensité. Un modèle de turbulence qui prend en compte le déséquilibre est nécessaire pour améliorer ce point. En dernier lieu, une nouvelle formulation hybride RANS/SGE est proposée, fondée sur la distance à la paroi en unités de paroi. La formulation est validée dans un canal bi-périodique et un premier essai est réalisé sur le cas de décollement de coin, mais d'autres analyses sont nécessaires avant que cette formulation ne soit fonctionnelle. / The present study aims at analysing turbulence modelling in Reynolds-averaged Navier-Stokes (RANS) simulations, in the context of turbomachinery flows, using large-eddy simulations (LES) as references. Two test cases are considered: a corner separation (CS) flow in a linear compressor cascade, and a tip-leakage (TL) flow of a single blade in a jet. Two constitutive relations, the Boussinesq constitutive relation and the quadratic constitutive relation (QCR), are investigated, with two versions of Wilcox's $k-\omega$ turbulence model. The studied constitutive relations rely on two hypotheses: an alignment hypothesis between the Reynolds stress tensor and a mean flow tensor, and an hypothesis on the turbulent viscosity. The alignment hypothesis is investigated using LES, where both the tensors are known independently, with an indicator built on the inner product of the tensors. The results are presented as probability density functions of the indicator value for the entire domain first, and then for three specific areas of interest: the inlet area, similar to a boundary-layer flow, an area of strong interaction between the flow and the walls (CS: passage area, TL: tip clearance) and an area of highly vortical flow (CS: separation wake, TL: tip-leakage vortex). The alignment hypothesis is rarely verified in any area for the Boussinesq constitutive relation. For the QCR, the results are improved for the inlet areas compared to the Boussinesq constitutive relation, but no significant improvement is found in the highly vortical regions. An improvement of the constitutive relation is needed in order to improve the RANS turbulence modelling. In contrast, the use of the turbulent kinetic energy and the specific dissipation rate appears quite correct to estimate the turbulent viscosity. The modelling of the RANS turbulent kinetic energy (TKE) budget equation is investigated through a term to term comparison with the resolved LES TKE budget equation. The LES presents a turbulence that is not at equilibrium, with the production and the dissipation not superimposed, and an important amount of transport. This differs from the RANS models, at equilibrium: the production and the dissipation are superimposed, with a small amount of transport. The development of a non-equilibrium turbulence model for RANS simulations could improve this aspect of turbulence modelling. Finally, a new hybrid RANS-LES formulation, based on the wall distance in wall units, is also proposed. It is validated on a bi-periodical channel flow, and a first attempt is made on the corner separation case, but further investigations are still needed for the model to be fully operational.

This work evaluates whether the branch of Reynolds Averaging in Computational Fluid Dynamics can be used to, based on real field measurements, find the source of the measured gas in question. The method to do this is via the adjoint to the Reynolds Averaged Scalar Transport equation, explained and derived herein. Since the Inverse is only as good as the main equation, forward runs are made to evaluate the turbulence model. Reynolds Averaged Navier Stokes is solved in a domain containing 4 cubes in a 2x2 grid, generating a velocity field for said domain. The turbulence model in question is a union of two modifications to the standard two equation k-ε model in order to capture blunt body turbulence but also to model the atmospheric boundary layer. This field is then inserted into the Reynolds Averaged Scalar Transport equation and the simulation is compared to data from the Environmental Flow wind tunnel in Surrey. Finally the adjoint scalar transport is solved, both for synthetic data that was generated in the forward run, but also for the data from EnFlo. It was discovered that the turbulent Schmidt number plays a major role in capturing the dispersed gas, three different Schmidt numbers were tested, the standard 0.7, the unconventional 0.3 and a height dependent Schmidt number. The widely accepted value of 0.7 did not capture the dispersion at all and gave a huge model error. As such the adjoint scalar transport was solved for 0.3 and a height dependent Schmidt number. The interaction between measurements, the real source strength (which is not used in the adjoint equation, but needed to find the source) and the location of the source is intricate indeed. Over estimation and under estimation of the forward model may cancel out in order to find the correct source, with the correct strength. It is found that Reynolds Averaged Computational fluid dynamics may prove useful in source term estimation. / Detta arbete utvärderar hurvida Reynolds medelvärdesmodellering inom flödessimuleringar kan användas till att finna källan till en viss gas baserat på verkliga mätningar ute i fält. Metoden går ut på att använda den adjungerade ekvationen till Reynolds tidsmedlade skalära transportekvationen, beskriven och härledd häri. Då bakåtmodellen bygger på framåtmodellen, måste såleds framåtmodellen utvärderas först. Navier-Stokes ekvationer med en turbulensmodell löses i en domän, innehållandes 4 kuber i en 2x2 orientering, för vilken en hastighetsprofil erhålles. Turbulensmodellen som användes är en union av två olika k-ε modeller, där den ena fångar turbulens runt tröga objekt och den andra som modellerar atmosfäriska gränsskiktet. Detta fält används sedan i framåtmodellen av skalära transportekvationen, som sedan jämförs med körningar från EnFlo windtunneln i Surrey. Slutligen testkörs även den adjungerade ekvationen, både för syntetiskt data genererat i framåtkörningen men även för data från EnFlo tunneln. Då det visade sig att det turbulenta Schmidttalet spelar stor roll inom spridning i det atmosfäriska gränsskiktet, gjordes testkörningar med tre olika Schmidttal, det normala 0.7, det väldigt låga talet 0.3 samt ett höjdberoende Schmidttal. Det visade sig att det vanligtvis använda talet 0.7 inte alls lyckas fånga spridningen tillfredställande och gav ett stort modellfel. Därför löstes den adjungerade ekvationen för 0.3 samt för ett höjdberoende Schmidttal. Interaktionen mellan mätningar, den riktiga källstyrkan (som är okänd i den adjungerade ekvationen) samt källpositionen är onekligen intrikat. Över- samt underestimationer av framåtmodellen kan ta ut varandra i bakåtmodellen för att finna rätt källa, med rätt källstyrka. Det ter sig som Reynolds turbulensmodellering mycket möjligt kan användas inom källtermsuppskattning.

Le développement de l'accès à l'espace s'inscrit dans le contexte scientifique et économique actuel comme un enjeu majeur de l'industrie et de la recherche. Un des objectifs principaux est d'augmenter la capacité et le confort de la charge utile pour réduire les coûts de transport vers l'espace. L'exploitation des données en vol du lanceur Ariane 5 a mis en évidence la présence de fluctuations de pression pouvant induire des efforts instationnaires repris par les vérins du moteur Vulcain. Ces efforts s'exercent dans la zone décollée du culot d'un lanceur normalement à l'axe de la poussée, et sont qualifiés de charges latérales. Cette observation conduit à l'étude de deux aspects. Dans un premier temps, la dynamique des écoulements massivement décollés d'arrière-corps, à haut nombre de Reynolds et en régime compressible est considérée au moyen de simulations numériques ZDES. Une analyse approfondie des champs instantanés, moyens et fluctuants est réalisée au moyen de post-traitements avancés. Notamment, le champ tri-dimensionnel de pression fluctuante dans la région du culot fait l'objet d'analyses spectrales (Fourier) et modales (DMD) massives. Ensuite, le contrôle des phénomènes potentiellement nuisibles au confort de la charge utile à savoir le buffeting culot et l'ovalisation de tuyère est abordé. L'analyse physique de l'écoulement sur une géométrie tri-corps permet la conception d'un dispositif de contrôle adapté. Enfin, l'effet de deux dispositifs retenus (quatre jets équirépartis en azimut sur le corps central et augmentation de la section de passage entre les trois corps) est évalué au moyen des outils d'analyse de la dynamique de la configuration non contrôlée.

Hot wire anemometers have been used in several wind velocity sensors deployed in Mars. They are based in keeping the temperature of a surface at a constant value, above the ambient. This is done by means of a heater controlled with an electronic system. The cooling rate of each point at the sensor surface can be used to calculate the wind velocity and direction. However, due to turbulent fluctuations, the cooling rate is not constant even in the case of constant velocity. Moreover, RANS simulations cannot estimate such fluctuations as they only provide an estimation of the averaged flow field. The goal of this work has been to estimate such fluctuations and the e↵ect they might have on the sensor readings. To do so, the turbulent cooling rate (Nusselt number) of a sensor with a generic shape, under the typical conditions to be find in Mars, has been simulated using high performance LES (Large Eddy Simulation) simulations and compared with RANS and URANS simulations.

To avoid the large amount of damage that floods can cause, spillway tunnels are used to control water levels. To ensure the safety of water transportation through spillway tunnels, the behaviour of the water throughout the tunnel is important to know. Physical experiments are time consuming and expensive, hence CFD simulations are a profitable option for investigating the performance of the spillway tunnel. In this project, simulations of water flow in a spillway tunnel were executed. A three dimensional model of the spillway tunnel in Zipingpu Water Conservancy Project was created in the software ANSYS Gambit. A coarse, middle and fine mesh with both hexahedral- and tetrahedral elements were also created for the model in ANSYS Gambit. The meshes were imported to ANSYS Fluent where the simulations, and a convergence analysis were made. The water flow was set to be described by the Reynolds-Averaged Navier-Stokes model, using the pressure solver, k-epsilon model and the VOF model. Physical experiments had previously been performed, and the simulated results were compared to these, in an attempt to find the parameters to replicate the experimental results to the greatest extent possible. The inlet velocity of the tunnel was known and the inlet boundary was set as a velocity inlet. The ceiling of the tunnel was set as a pressure inlet, the floor and walls were set as wall, and the outlet was set as pressure outlet. The simulated results showed similar behavior as the experimental results, but all differed from the experimental results. The grid convergence index, estimating the results' dependency on the mesh was 6.044 %. The flow was analyzed, and where the flow had unfavorable characteristics, such as a high cavitation number, the geometry of the spillway was altered in ANSYS Gambit to investigate if an improved geometry for the spillway tunnel could be found. The water flow in the revised geometry was simulated in ANSYS Fluent, and results showing flow with lower cavitation numbers was found.

Ce travail de thèse a été consacré au développement d'outils numériques pour aider à la compréhension, par simulation numérique, de l'aérodynamique d'un inverseur de poussée à portes (écoulement turbulent compressible). Dans cette étude, nous avons adopté deux approches complémentaires : résolution des équations de Navier-Stokes moyennées RANS (stationnaires) ou filtrées LES (instationnaires). Dans un premier temps, les divers phénomènes physiques présents dans un inverseur de poussée sont décomposés et étudiés séparément. On simule ainsi successivement :<br /> (1) un écoulement décollé (marche descendante - calcul RANS),<br /> (2) un écoulement cisaillé libre (couche de mélange plane - calculs RANS et LES), <br />(3) des écoulements présentant de fortes courbures des lignes de courant (conduite rectangulaire courbée (calcul RANS) et couche de mélange courbe - calculs RANS et LES). Ensuite, deux configurations simplifiées d'inverseurs de poussée à portes sont étudiées numériquement par simulations RANS et LES.